화학공학소재연구정보센터
Applied Surface Science, Vol.257, No.19, 8241-8245, 2011
Surface and bulk phenomena in the process of hydride formation in thin films of rare earth metals: Comparison between terbium and europium
Work function changes Delta Phi caused by H-2 interaction with thin terbium and europium films deposited on glass under UHV conditions were correlated with hydrogen uptake and electrical resistance R, measured in situ. For both metals, the course of Delta Phi (H/Me) at room temperature confirms the change in charge-transfer direction on the surface during hydride formation. As a result, the hydrogen adsorbate's nature is changed from positively polarized (precursor state) to negatively polarized adspecies. The hydrogen behavior is significantly different at low temperature due to the formation of the surface ordered low-temperature phase (alpha') with positively polarized hydrogen adspecies. This phase, strongly inhibiting penetration of hydrogen into the bulk, is stable up to 100 K for terbium and 160 K for europium. Increasing temperature above these values resulted in additional large absorption of hydrogen. Moreover, differences in the course of R(H/Me) are clearly noticeable between the investigated metals. The resistance of thin TbHx (x similar to 3) films in our experiments did not exceed 1 k Omega, however transition of thin metallic europium film into EuHy (y similar to 2) increased the resistance up to 10 M Omega. This dissimilarity in electrical behavior can be explained by the coexistence of two factors which are different for the two metals in question: the phase relation of hydrides and the response of thin film to stress generated during hydride formation. (C) 2011 Elsevier B. V. All rights reserved.